1. Field of the Invention
The present invention relates to a method and apparatus for encoding security status information.
2. Description of the Prior Art
Low rate personal wireless networks are used with small devices with transmission speeds of up to 250 kilobits per second. These devices typically have severe power constraints as they are operated on batteries. In many of these devices, such as battery-operated sensors, remote controls, car door openers and light switches, it is necessary to have long battery life. If the batteries die too quickly then the replacement cost can be equal to the cost of the product itself.
It is also desirable to have secure communications between such constrained devices to prevent abuse of the system. One technique is to encrypt data being sent between the devices. Encryption mathematically transforms the transmitted information using a secret key known only to the two parties who are communicating. Without the key, the message is unintelligible. However, this requires overhead in the message structure in order to allow the recipient to decrypt the data. The sender must indicate which key it has used, which algorithm it has used to encrypt, and input parameters of the encryption algorithm such as a counter.
Usually, a frame counter is used as one of the input parameters for freshness in the encryption. Freshness means that the parameters change for each communication and are thus not reused. One type of encryption called a block cipher breaks up a message into parts (blocks) of a fixed size. Various block ciphers are known such as DES (Data Encryption Standard) and AES (Advanced Encryption Standard). Block ciphers often use an input block as a seed when used in stream-cipher mode. This input block should not repeat in order to maintain data freshness and data confidentiality. In one approach, a frame counter and a key identifier are used as the input block and are indicated in the message that is sent. In addition, each message usually includes a sequence counter that is not used for security but rather to match the sending of a message with the acknowledgement thereof by the recipient. These messages typically include a data portion referred to as the payload which is about 20 bytes. Accordingly, a five byte overhead for security information represents a 25% overhead.
The amount of data transferred between such constrained devices is one of the principal factors in their battery life. Accordingly, it is desirable to reduce the amount of information transferred.
However, in order to maintain the security of the underlying encryption methods, the number of bits in the frame counter should not be reduced.